rados/singleton-nomsgr/all/objectstore: clean up

ceph_test_objectstore scribbles in $CWD.  Keep it inside $TESTDIR and
clean up when we are done.

Fixes: #8395
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 5be30f0d9bba46684ecf37310a6d09ae4f0ac528)